I. Field of the Invention
The present invention relates to the transfer of a paper sheet between sections, or between elements of a section, such as the individual presses in a press section, of the papermachine on which it is being manufactured. Specifically, the present invention is a transfer belt designed both to carry a paper sheet through a portion of a papermachine, so as to eliminate open draws, wherein the paper sheet receives no support from a carrier and is susceptible to breakage, from the machine, and to release the sheet readily to another fabric or belt at some desired point.
II. Description of the Prior Art
The prior art is replete with proposals for eliminating so-called open draws from papermachines. By definition, an open draw is one in which a paper sheet passes without support from one component of a papermachine to another over a distance which is greater than the length of the cellulose fibers in the sheet. All such proposals for eliminating open draws have as their object the removal of a major cause of unscheduled machine shut-down, the breakage of the sheet at such a point where it is temporarily unsupported by a felt or other sheet carrier. When disturbances in the normally stable flow of paper stock occur, the likelihood of such breakage is quite strong where the unsupported sheet is being transferred from one point to another within the press section, or from the final press in the press section to the dryer section. At such points, the sheet usually is at least 50% water, and, as a consequence is weak and readily broken. At present, then, an open draw will place a limitation on the maximum speed at which the papermachine may be run.
The prior-art proposals for eliminating open draws include some form of transfer belt to carry and support the paper sheet between components of the papermachine. In so doing, the transfer belt may have to carry out several of the following separate functions:
a) to take the paper sheet from a press roll or press fabric (felt); PA1 b) to carry the paper sheet into a press nip; PA1 c) to work cooperatively with a press fabric in the press nip to de-water the paper sheet; PA1 d) to carry the paper sheet out of the press nip; PA1 e) to repeat functions b) through d) as necessary where the transfer belt carries the paper sheet through more than one press; and PA1 f) to transfer the paper sheet to another fabric or belt, such as, for example, a dryer fabric.
As will be discussed below, there are specific problems associated with each of these transfer belt functions.
Transfer belts are shown in a number of issued U.S. patents. For example, U.S. Pat. No. 4,483,745 shows press arrangements which may be either the typical paired roller press or a long-nip press. In the press arrangements illustrated, the paper sheet is sandwiched between a press fabric and a looped, endless, and impermeable belt which is relatively smooth and hard, so that the paper sheet may follow the belt upon leaving the press nip without being rewet by a press fabric or other permeable belt. This arrangement utilizes the fact known to papermakers that the paper sheet will follow the surface to which it may be most strongly bonded by a thin, continuous water film, and for this reason will follow a smooth, impervious surface rather than a coarser surface when the two are separated in a papermachine.
Little detail is provided, however, on the structure of the belt itself beyond describing it as having a smooth upper surface with a smoothness and a hardness or density generally similar to a plain press roll cover. The belt surface is said to preferably have a hardness in the range of between 10 and 200 P&J (Pusey & Jones Hardness Scale). No recognition is given to the difficulty which would actually be encountered in attempting to remove a wet paper sheet from the surface of such a belt in a papermachine.
U.S. Pat. No. 4,976,821 shows another press configuration with no open draws. In the press sections described and illustrated therein, there are two successive press nips for dewatering a paper sheet, which passes in a closed draw between the nips. The paper sheet is also transferred from the last press nip of the press section to the drying section in a closed draw by a substantially non-water receiving transfer fabric. The paper sheet is removed directly from the surface of the substantially non-water receiving transfer fabric, and placed onto a dryer fabric by means of a suction roll.
In contrast to the belt shown in the '745 patent, the substantially non-water receiving transfer fabric shown in the '821 patent generally is relatively impervious, and may, for example, be a fabric produced by impregnating a press fabric with an appropriate plastic material. That is to say, it is relatively impervious when compared to an unimpregnated press fabric. As such, however, the '821 patent teaches that the fabric may still to some extent participate in the dewatering of the paper sheet in the press nip, so that the paper produced may be more symmetric in density and surface smoothness than that produced when the transfer belt is smooth and impermeable. While it is said to be easier to remove the paper sheet from the surface of such a transfer fabric, there is no recognition given to the problems actually associated with the use of a transfer fabric of this variety on a papermachine. In actual use, such a sheet transfer belt, designed to function with a low, constant porosity, will eventually meet with failure. Fine particles from the paper stock, such as cellulose fines, fillers, resins, and "stickies", rapidly fill the pores in such a belt. High-pressure water jet showering, the standard method to keep fabrics and felts clean and open on a papermachine, is not efficient on a fine-porous structure such as the one described in this '821 patent.
In general, and referring to the various functions of a transfer belt identified above, where the transfer belt removes the paper sheet from a press roll, a procedure rarely used in practice, it must overcome the strong adhesion the paper sheet will normally have for the roll, which may be very smooth. In the in-going side of a press nip, the paper is squeezed until it becomes fully saturated, at which point water will start to move out from the sheet into the water receptor, the press fabric. As a consequence, there will always be a water film, perhaps partly broken, at the interface between the roll surface and the paper sheet. This film has to be broken before the paper sheet may be reliably transferred from the roll to the transfer belt.
Where the transfer belt carries the paper sheet into a press nip, a belt having a non-air-permeable paper-side surface is generally preferred to one which is permeable. A transfer belt which may be permeable to some extent is described in the '821 patent discussed above. Others are described in U.S. Pat. Nos. 4,500,588 and 4,529,643, which will be discussed below. The disadvantage associated with the use of permeable or semi-permeable transfer belts is the risk of blowing of the paper sheet at the entrance of the press nip, as a result of air being forced out of the porous belt being compressed, or even through the transfer belt from its backside by a press roll.
In the press nip, the transfer belt must work cooperatively with a press fabric to dewater and to densify the paper sheet. As a consequence, the surface topography and compression properties of the transfer belt are critical for producing a paper sheet with a smooth, mark-free surface. Because, as is well known to those skilled in the art, even a high quality, well-broken-in press fabric may provide a very non-uniform pressure distribution in the nip, a transfer belt having a smoother and harder paper-side surface than the press fabric will provide a more uniform pressure distribution to the paper sheet being dewatered, and will impart a smoother surface to the sheet.
Further, a transfer belt with suitable compression properties can in effect lengthen the press nip to increase the time the paper sheet is exposed to pressure and to allow more time for water to leave the paper sheet under a given press load. In addition, a transfer belt with a paper side impermeable to water and air will contribute to the dryness of the paper sheet by eliminating the possibility of rewet after the press nip, as may occur when a conventional press fabric carries the paper sheet out of the nip.
Clearly, a transfer belt must be designed with the understanding that it will work cooperatively in the nip with a press fabric as a functional pair in order to provide high dewatering efficiency and high paper quality.
Referring again to the various transfer belt functions identified above, the transfer belt should carry the paper sheet out of the press nip. That is to say, more precisely, the paper sheet should adhere to the surface of the transfer belt upon exiting the nip, as opposed to following the press fabric out of the nip and then moving over to the transfer belt after the nip. Not only does the latter permit rewet while the paper sheet remains in contact with the press fabric, but the moving of the paper sheet over to the transfer belt after leaving the press nip would also constitute an open draw, the very problem the transfer belt is intended to eliminate. Such a situation can lead to blistering or some other deformation of the paper sheet. A good adhesion of the sheet to the transfer belt on the exit side of the nip is even more important in press configurations where the belt is run in the top position and the sheet is to be transferred on the underside of the belt. As before, the paper-side surface of the transfer belt should be neither water-absorbent nor waterpermeable, so that rewet of the paper sheet by the transfer belt may be avoided.
Where the transfer belt carries the paper sheet through more than one press, the stability of the transfer belt will become an important factor. The speed of consecutive presses in a press section can never be absolutely synchronized, and, normally, will increase somewhat downstream in the section. Under such conditions, the transfer belt must be able to carry the paper sheet without blowing, blistering, or drop off. In addition, the transfer belt itself must be of a durable design, capable of enduring the backside wear and high shear forces, which would attend its use through more than one press, without rapid degradation.
The final, and most critical, function of the transfer belt is to effect a correct transfer of the paper sheet to the next section of the papermachine. In many applications, this will be a transfer to the first fabric in the dryer section. It is preferred that this first fabric should be of a design suitable for both paper drying and for the closed transfer of the paper sheet.
A typical dryer fabric in the first drying position may be a woven, all-polyester monofilament fabric. Fabrics used in first drying positions normally have a low airpermeability and a smooth, fine paper side. Hence, the surface to which the transfer belt is to transfer the paper sheet may initially consist of smooth, hydrophobic monofilament knuckles.
The transfer from the transfer belt to the first dryer fabric should be carried out with as low a contact pressure as possible in order to avoid the marking of the paper sheet by the knuckles. Since the dryer fabric is air-permeable, vacuum may be used to assist the transfer of the paper sheet from the transfer belt. In order to avoid the marking of the paper sheet by the knuckles of the first dryer fabric, the vacuum level used at the transfer point must be as low as possible. It follows, then, that the transfer belt must readily release the paper sheet at the transfer point so that the vacuum level required may be kept at a minimum level.
As noted above, transfer belts of several varieties are known in the prior art. For example, in U.S. Pat. No. 5,002,638 a wet paper web is supported on a press fabric and passed through the nip between cooperating press rolls to extract water from the web. The press fabric, supporting the paper web, then travels through a span of distance and is passed around a heated dryer roll in the dryer section with the felt being interposed between the heated roll and the paper web. The press fabric is thus heated and insulates the paper web from the high temperature roll. The paper web is then separated from the press fabric and travels around the remaining dryer rolls in the dryer section, while the heated press fabric is returned to the nip into position to support the wet paper web.
The disadvantage following such an approach is considerable rewet of the paper sheet in the span between the press nip and the heated dryer roll, because the transfer belt is literally a press fabric. Further, such a transfer belt is not hard enough to replace a smooth roll surface in late presses on a publishing-grade papermachine. In short, the only reasonable application for a transfer belt of the variety shown in U.S. Pat. No. 5,002,638 is in slow machines producing heavy paper grades.
The use of modified press fabrics as transfer belts is shown in several U.S. patents. For example, U.S. Pat. No. 4,500,588 shows a conveyor felt for conveying a paper web through a press section of a paper machine. The conveyor felt is, with the exception of the surface portion of the fiber batt layer facing the web, filled with a filling material so that the felt is completely air-impermeable and has a chamois-like surface. Such a surface is, because of its fibrous character, sensitive to soiling by sticky materials, and the chamois-like structure is sensitive to wear and difficult to maintain.
In U.S. Pat. No. 4,529,643, a press felt for conveying a paper web through a press section of a papermachine is shown. It comprises a support fabric formed of a yarn structure and a fibre batt layer, formed of fibers and needled to at least one side of the support fabric. The support fabric and the fiber batt layer are filled with a filling material, preferably from the surface facing the paper with a rubber or resin emulsion, so that the press felt remains slightly air permeable.
Belts of the variety shown in these two patents have exhibited sheet drop-off upon exit from the press nip. The cause of this sheet drop-off is related to the inability of the porous surface of such a belt to permit a thin, continuous water film to form between its surface and a paper sheet in the press nip, and to maintain such a water film long enough to ensure that the paper sheet will follow the belt rather than the press fabric upon exit from the press nip. In addition, it is difficult to maintain the porosity of this variety of belt at a constant value, as material from the paper stock gradually fills the pores. High-pressure showers have not proved effective on the microporous structure of the surface of such belts, and may actually destroy the belt surface.
Finally, non-compressible, coated belts, such as those used as long nip press (LNP) belts, have also been tested for use as transfer belts. A belt of this kind is shown in Canadian Patent No. 1,188,556, and comprises a base fabric which is impregnated with a thermoplastic or thermosetting polymeric material. The belt is of uniform thickness, and has at least one smooth surface. While the belt performs in a superior manner in its intended position on a long nip press, all attempts to use it as a transfer belt have failed, as the belt could not be arranged to release a paper sheet to a dryer fabric. This is believed to result from the failure of a thin film of water between the impermeable belt and the paper sheet to break up into droplets, allowing the paper sheet to be separated from the transfer belt.
The present invention provides a long-sought solution to these difficulties in the form of a transfer belt not susceptible to the shortcomings of the prior-art transfer belts discussed above.